Unregister Nav Menu with fallback?

You can filter 'wp_nav_menu_args', set an invalid theme_location and the magic function __return_false as fallback:

add_filter( 'wp_nav_menu_args', 'override_parent_args' );

function override_parent_args( $args )
{
    if ( 'header-menu' === $args['theme_location'] )
    {
        $args['fallback_cb']    = '__return_false';
        $args['theme_location'] = 'ticky_tacky';
    }

    return $args;
}